The overall objective of this proposal is to define in molecular and biochemical terms the events regulating the transcription and replication of vesicular stomatitis virus (VSV) and its defective interfering particles (DI). Six specific areas will be examined. All but the last one of these are related to a recently discovered VSV polymerase variant, pol Rl, which appears to be mutated in the regulation of transcription versus replication. These research aims include characterization of pol Rl products in vitro and in vivo, identification of the mutated polypeptide(s) in pol Rl, definition of the regulatory role of the in vitro transcriptase inhibitor, characterization of pol Rl interaction with DI templates, and isolation of additional pol R variants. The last research topic is concerned with testing the role of polymerase mutations in the establishment and/or maintenance of VSV persistence in tissue culture.